HEAVY OILS (LUBRICATING OILS) 355 In the second case a preliminary test is first made. The oil is placed in a test-tube through the stopper of which passes a thermometer, and cooled in a freezing mixture ; from time to time the tube is withdrawn for an instant and inclined, so that the temperature at which the oil begins to solidify may be discovered. A saline solution with a freezing point a little lower than this temperature is then chosen and the oil kept in the apparatus referred to above at the temperature found in the preliminary trial. At the end of the time the test-tube is removed from the solution and inclined so that an idea may be obtained of the degree of thickening; the latter may also be estimated by noting the adhesion of the oil to a glass rod when this is withdrawn. Supercooling of the salt solution is avoided by scraping the congealed parts from the walls of the containing vessel or by withdrawing the vessel itself for an instant from the freezing mixture. If the cooled oil is heated and another examination made of the behaviour at low temperatures, the result may be different from that of the first test; such difference may be due to the variations of temperature to which the oil is subjected during transport and storage. It is therefore useful to carry out the test on the oil, first as received and then after it has been heated to 50° for ten minutes and subsequently cooled for 30 minutes in a water-bath at 20°. With mixtures of mineral and fatty oils, the cooling should be protracted for 4-10 hours, one test being made without stirring the oil and the other with stirring every 15 minutes. 9. Test of Fluidity at Low Temperatures.—This is effected when required by pipetting the oil into a U-tube of definite diameter (usually 6 mm.), cooling the latter in a cooling mixture, and then measuring the change of level produced by applying at one side, for a definite period (generally i minute), a known pressure, e.g., 50 mm. of water, by means of a water manometer. The number of mm. measuring the change of level represents the degree of fluidity of the oil at the temperature of the experi- ment. 2. Chemical Tests 1. Detection of Water and Solid Substances.—These are usually recognised by the appearance and are investigated as with crude petroleum. Water may also be detected by heating the oil in a test-tube for about 15 minutes : if water is present froth is formed and drops of water appear in the cold part of the tube. 2. Determination of the Acidity.—Acidity is due either to mineral acids (sulphuric acid) introduced during refining or to organic acids. (a] Acidity due to mineral acids may be detected by shaking 50-100 grams of the oil with double the quantity of distilled water, allowing the aqueous layer to separate, filtering it through a moist filter-paper and testing about 30 c.c. of the filtrate with a few drops of methyl orange solu- tion (0-03%) : when mineral acid is present a red coloration is obtained. (&) The acidity due to organic acid is determined differently according as the oil is pale or dark.1 1 When the test for inorganic acidity made as in (a) gives a positive result, the oil should be first subjected to washing with hot water.l2, 22-5.